1. Field of the Invention
The present invention relates to data transmission in which a sequence of symbols, viz. signals of selectable phase and amplitude are selected from a range of discrete phase/amplitude combinations.
2. Description of the Prior Art
Examples of prior systems capable of transmitting fractional bits/baud and/or for modulation shaping a sequence of symbols of selectable amplitude and phase so as to control average power of transmitted signals are described, for example, in U.S. Pat. No. 5,214,672--Eyuboglu et al and U.S. Pat. No. 5,150,381--Forney, Jr., et al.
Prior art techniques have included quadrature amplitude modulation (QAM) where groups of q bits are assembled and one or more bit in each group is coded (e.g. by a convolutional or block code) to produce an augmented group having at least q+1 bits. For each such augmented group, a symbol is selected for transmission from a signal point constellation having more than 2.sup.q points using a variable mapping controlled by generating, for each augmented group, power signals representing the signal power corresponding to each of a plurality of alternative mappings of the group. Such power signals are decoded by a Viterbi decoder to determine a mapping for that group that substantially minimizes the time averaged power of the transmitted signals.